Chapter 22 

 THE SEPARATE FUNCTIONS OF RECESSIVE AND DOMINANT 

 ALLELES IN ADAPTATION TO PANTOTHENATE DEFICIENCY 



The classification of genetically different yeasts into dominant 

 "synthesizers" and recessive "non-synthesizers" of the different 

 B vitamins is merely a relative distinction, for the so-called "non- 

 synthesizers" eventually grow in a test medium deficient for the 

 vitamin in question. In an investigation of the effect of environment 

 and competition on the adaption of cultures carrying the recessive 

 allele it was found that the adaptation occurred in two steps: (a) A 

 primary adaptation during which the recessive ("non-synthesi- 

 zer") grew at a low rate presumably by sjmthesizing the absent vi- 

 tamin, until the population attained a relatively high density, (b) A 

 secondary adap tat ion initiated by the mutation of one or more 

 recessive cells to the dominant so-called "synthesizer". Small a- 

 mounts of added vitamin were effective in reducing the time re- 

 quired to complete these steps. 



In high concentrations of pantothenate the dominant ("sjmthesi- 

 zer") cells were unable to compete with cells carrying the reces- 

 sive allele and, therefore, adaptations were only apparent in low 

 concentrations of pantothenate. This latter observation is consis- 

 tent with the observations of Lwoff, who pointed out that the "in- 

 ability" to synthesize a specific nutrilite may be a selective advan- 

 tage in high concentrations of the specific substance. 



THE EFFECT OF THE ENVIRONMENT 



ON ADAPTATION TO A 



DEFICIENT MEDIUM 



Microorganisms in an "unfavorable" environment frequently be- 

 come adapted and survive and grow in that environment. Bacteria 

 exposed to antibacterial agents such as penicillin (Demerec 1945a, 

 b) or sulfonamide (Oakberg and Luria 1947) acquire a resistance 

 which is transmitted. In many cases, the reason for the increased 

 resistance is unknown. In some cases it is due to increased syn- 

 thesis, as for example, of p-aminobenzoic acid in Staphylococcus 

 aureus resistant to sulfonamide (Landy et al 1943) and increased 

 synthesis of pantothenate in pantoyl taurine resistant Corynebacteri- 

 um diphtheriae (Mcllwain 1943). 



Mutant strains of Neurospora crassa unable to grow in the ab- 

 sence of a specific nutrilite will sometimes adapt while being grown 



22-1 



